Simple anti-reflection (AR) coatings, such as those disclosed in U.S. Pat. No. 2,478,385 issued Aug. 9, 1949 to Gaiser, and U.S. Pat. No. 3,185,020 issued May 25, 1965 to Thelen, include a three layer structure mounted on a substrate in which a high index layer, which has an optical thickness of a half of the center wavelength of a given bandwidth of light, is sandwiched between two lower index layers, which each have an optical thickness of one quarter of the center wavelength. The inner ¼ wavelength layer index matches the substrate to the center ½ wavelength layer, while the outer ¼ wavelength layer index matches the center ½ wavelength layer to the air. The aforementioned structure is known as the QHQ structure.
Improvements to the basic QHQ structure have been developed to improve performance, such as the one disclosed in U.S. Pat. No. 3,432,225 issued Mar. 11, 1969 to Rock, in which the inner low index, ¼-wavelength layer is replaced by a simulated ¼ wavelength layer comprising one high-index layer, which has an optical thickness of one eight the center wavelength, and one low-index layer, which also has an optical thickness of one eight the center wavelength.
More recent developments include the addition of a transparent electro-conductive layer to the basic QHQ structure for a variety of reasons, e.g. to provide electromagnetic interference (EMI) shielding, and for thin film heater applications. Conventionally, as disclosed in U.S. Pat. No. 4,422,721 issued Dec. 27, 1983 to Hahn et al, U.S. Pat. No. 5,667,880 issued Sep. 16, 1997 to Okaniwa, U.S. Pat. No. 6,532,112 issued Mar. 11, 2003 to Chu, and U.S. Pat. No. 6,586,101 issued Jul. 1, 2003 to Chu, the transparent electro-conductive layer is formed from indium tin oxide (ITO) and is positioned outside the QHQ structure, i.e. next to the substrate or on the outer layer. Unfortunately, the reflectance performance of an ITO-based electro-conductive AR coating is not as good as the conventional non-conductive AR coating, as illustrated in FIG. 1, in particular near the lower and upper ends of the visible bandwidth, i.e. 440 mm and 675 nm, respectively, because the index of refraction of the ITO is too low, e.g. <2, resulting in a high index to low index ratio of <1.45.
Indices of ITO are a function of the coating rate or the oxidation level of the film, i.e. the slower the rate becomes the higher the indices throughout the visible range of the spectrum. If the coating rate is slowed down enough, ITO will start transmitting well beyond 1,000 nm, but the resistivity will increase rapidly as the oxidation level increases. Accordingly, an effective EMI shielding AR coating cannot be build providing a sheet resistance of less than 20 ohms/sq with ITO films.
Accordingly, conventional ITO-based HEA coatings can not be used for avionics instruments, e.g. for display cover glass, which are typically required to meet the MIL-C-14806A 1974 amendment that requires the maximum reflectance at 440 nm and 657 nm to be 0.55%, and the bandwidth ratio at 0.55% R to be at least 1.534.
An object of the present invention is to overcome the shortcomings of the prior art by providing an electro-conductive AR coating that incorporates a transparent electro-conductive layer with a refractive index >2 into the QHQ structure resulting in a high index to low index ratio of between 1.45 and 1.55 with a sheet resistance of less than 20 ohms/sq.